The invention relates to a curable composition suitable for coating ribbon matrices and glass surfaces, specifically, coating optical waveguides such as optical fibers, the coating compositions having a faster cure time to achieve greater line speeds.
Optical fibers made from drawn glass have been used as a reliable transmission medium in telecommunications cables. Glass optical fibers are widely used because they have the ability to carry large amounts of information over long distances.
To facilitate these long-distance transmissions, optical fiber waveguides have been coated with plastic compositions of various materials in order to protect the fiber and increase its tensile strength. Generally, to accomplish this optical glass fibers are frequently coated with two superposed coatings. The coating which contacts the glass is a relatively soft, primary coating that must satisfactorily adhere to the fiber and be soft enough to resist microbending especially at low service temperatures. The outer, exposed coating is a much harder secondary coating that provides the desired resistance to handling forces yet must be flexible enough to enable the coated fiber to withstand repeated bending without cracking the coating.
Optical fiber coating compositions, whether they are inner primary or single coatings, generally comprise before cure an ethylenically-unsaturated monomer or oligomer dissolved or dispersed in a liquid ethylenically-unsaturated medium and a photoinitiator. The coating composition is typically applied to the optical glass fiber in liquid form and then exposed to actinic radiation to effect cure.
In practice, the most commonly used coatings have been derived from acrylates. The most widely used acrylates are those which are capable of ultraviolet radiation curing at high speed since the coatings are normally applied immediately after the glass fiber has been drawn from the molten state. Typical of such acrylates are mono- or difunctional (meth)acrylate terminated monomers and oligomers. The outer coatings are most often urethane-acrylate or epoxy-acrylate copolymers which also can be cured by ultraviolet radiation (See Shustack et al, U.S. Pat. No. 6,048,911; Barraud et al, U.S. Pat. No. 5,650,231).
Coatings are applied to the fiber in-line during fiber drawing. As the state of fiber drawing technology has allowed for increased draw speeds to effectuate longer and thinner optical fibers, however, the need for coating compositions that can cure at faster rates coincident with the faster draw speeds has become more urgent. Thus, as draw speeds have increased, a need has developed for materials that cure at faster rates than is currently available with traditional technology.
U.S. Pat. No. 4,663,185 to Eckberg, et al. describes a method of making a UV curable acrylated polymers by blending difunctional acrylate monomers of up to 50 wt % of the total composition to result in a hard, glassy coating to enhance abrasion resistance.
U.S. Pat. No. 4,968,116 to Hulme-Lowe, et al. describes a cladding composition using polyfunctional acrylates being difunctional or higher, at levels ranging from 2-35% in order to crosslink the resin to produce a hard coating.
U.S. Pat. No. 5,188,864 to Lee, et al. describes adhesion promoters for UV curable siloxane compositions that may contain up to 10 wt % multifunctional acrylates being difunctional or higher.
U.S. Pat. No. 6,001,913 to Thames, et al. describes a UV curable, high-gloss coating formulation which uses difunctional acrylates without the use of volatile organic components (VOC). Such coating compositions are poured to approximately a 2 mil thickness.
The present inventors have found that a liquid, non-crosslinked, UV curable composition having an decreased cure time can be provided by adding multifunctional low molecular weight acrylates to a composition comprising a radiation curable oligomer, a free radical photoinitiator, and a reactive diluent.